Conventionally, the organic EL films of an organic EL device have a lamination structure of a plurality of layered organic material layers. The organic material layers includes an organic light-emitting layer and also a material layer having a hole-transporting capability such as a hole injection layer, a hole transport layer or the like, as well as a material layer having an electron transporting capability such as an electron transport layer, an electron injection layer or the like. The electron injection layer may include an inorganic compound such as alkaline metal, alkaline earth metal or a compound which contains those of electron donating materials or only one electron donating martial, or may be made of a mixture of such an inorganic compound and an organic compound.
When an electric field is applied to the multilayered organic EL film of the organic light-emitting layer and electron or hole transport layers etc. being layered, then holes are injected from the source electrode as well as electrons are injected from drain electrode and these are recombined in the organic light-emitting layer so that excitons are generated. When the excitons return from an excited state to a ground state, light is emitted. In order to improve a luminous efficiency of the device, it is important to efficiently move electrons or the like carriers to the interface of the light emitting layer. The organic active light-emitting device also employs the multilayered structure utilizing organic compounds having charge-transporting properties i.e., electric charge transporting organic compounds.
It is well known that the organic EL device with the organic light-emitting layer is capable of a low voltage operation, but it is driven with a higher driving voltage in comparison with a light emitting diode or the like. There is a problem that a driving voltage of a phosphorescence organic EL device with a high quantum efficiency of light emission is higher than that of a fluorescence organic EL device.
Generally it is possible to reduce a driving voltage by setting to decrease the thicknesses of organic layers in the organic EL device. In this case there is increase in occurrence of defective electricity between the electrodes i.e., anode and cathode of the device, resulting in decrease of yields on products of the organic EL devices. An attempt to resolve such a problem has been made in a manner that a dopant is introduced into an adjacent layer to the electrode, i.e., a charge transport layer to raise the conductivity of the charge transport layer. Alkaline metal or alkaline earth metal and an organic compound may be used for the dopant doped into the charge transport layer through a co-evaporation so as to reduce efficiently a driving voltage of the organic EL device. However, since these alkaline metal and alkaline earth metal have deliquescence and absorbency, it is difficult to handle them. Especially alkaline metal and alkaline earth metal such as metal cesium etc. having a low work function exhibit a high doping efficiency, but it is very dangerous to deal with the metal cesium in the air due to instability thereof. In view of the instability, it has been confirmed that a similar effect may be obtained by using the salt of alkaline metal or alkaline earth metal and an organic compound for the dopant.
More recently, there have been suggested a phosphorescence organic EL device with a high power efficiency and a display and an illuminator that employ the same. This organic EL device has at least two layers of a hole blocking layer and an electron transport layer between a light-emitting layer containing at least one kind of phosphorescence emission dopant and at least one kind of host compound and a cathode, wherein the hole blocking layer contains an organic compound having triplet state transition energy of 2.8 eV or more and metallic atoms or ions having a work function of 2.9 eV or lower. (See Patent Literature 1).
It is valid to provide a charge injection layer to enhance a light-emitting efficiency of the organic EL device. Furthermore there is a demand for an organic EL device being capable of being driven for a long time continually with a higher light-emitting efficiency, and, a life-prolonged organic EL device is needed.
For the purpose to provide a stable organic EL element capable of being driven by a low voltage during a long-time storage without deterioration in luminance, there have been suggested another organic EL device. This organic EL element includes organic layers disposed between an anode and a cathode, in which at least one layer of the organic layers contains a metal salt, and further includes a diffusion preventing layer to suppress diffusion of metallic ions of the metal salt. (See Patent Literature 2)
Patent Literature 1: Japanese Unexamined Patent Publication No. 2007-180277
Patent Literature 2: Japanese Unexamined Patent Publication No. 2007-088015